The document of [CreateDCA][1] says:
> If lpszDriver or lpszDevice is DISPLAY, the thread that calls
> CreateDC owns the HDC that is created. When this thread is
> destroyed, the HDC is no longer valid. Thus, if you create the HDC
> and pass it to another thread, then exit the first thread,
> the second thread will not be able to use the HDC.
So this change introduces GPrivate to fix potential problem.
This also fixes the problem caused by accessing the global
variable DC directly, which makes some early call to Pango functions
fail.
(e.g., failure of calling pango_win32_font_description_from_logfontw
from _get_system_font_name in GTK.)
[1]: https://docs.microsoft.com/windows/win32/api/wingdi/nf-wingdi-createdca

Use a scaled down version of the GTK and GLib CI set ups, with a custom
Docker image and multiple stages. The CI pipeline now generates a JUnit
report, which integrates with the GitLab merge requests; builds the
documentation as a separate stage; and publishes the API reference for
the master branch at the end.

Do not use homegrown assertion and failure macros, when GTest has
perfectly functional API to deal with all the cases Pango covers.
Using GTest also allows us to rely on appropriate formatting for
results and logging.

`get_option('buildtype')` will return `'custom'` for most combinations
of `-Doptimization` and `-Ddebug`, but those two will always be set
correctly if only `-Dbuildtype` is set. So we should look at those
options directly.
For the two-way mapping between `buildtype` and `optimization`
+ `debug`, see this table:
https://mesonbuild.com/Builtin-options.html#build-type-options